Recently, as an example of low flow high head pump demanded in the market, a centrifugal pump in which impellers are provided in a multi-stage arrangement along a coaxial rotating shaft is being used to achieve a high head without increasing an outer diameter of the pump (see, for example, Patent Document 1).
In this configuration, energy is delivered to liquid by the impellers when the liquid is drawn sequentially into each of the impellers that are installed in the multi-stage arrangement. Thus, the discharging pressure is increased to achieve a high-head pumping.
(Patent Document 1)
Japanese Patent Application Publication No. 2001-65484
However, the above-described centrifugal pump is configured such that liquid drawn via an inlet port is discharged outwardly by a centrifugal force generated by a rotation of each of the impellers. Therefore, to increase the discharging pressure, it is necessary to minimize the leak of liquid that discharges from outlet ports of the impellers.
However, in the conventional centrifugal pump configured such that the impellers and a rotor in a motor part having permanent magnets are rotated together about a rotation support shaft installed in a casing, a gap exists between the casing and an outer peripheral side of the rotor. Therefore, a high pressure fluid discharged from the impellers may leak through the gap to thereby increase a loss of the fluid due to the leakage.